As is well known, in order to present accidents due to electric shock a leakage current protector (or a safety cut-out) may be used, a leakage current protector usually comprising a current transformer, amplifier, solid state switch, relay, direct current power supply and so on. When the current occurs from an alternating current power supply to ground, the voltage across the current transformer is proportional to the leakage current. After amplification by an amplifier the voltage is fed to a solid state switch. When the leakage current to ground is increased to a rated value, the solid state switch conducts, current passes through the coil of a relay connected in series between a DC power supply and the solid state switch, and the relay operates to interrupt the AC power supply, therefore safety is assured. The current to ground for the operation of the protector may be shock current through the human body or leakage current to ground from equipment and/or conductors. Under good conditions the leakage current of power lines is generally not over about 15 ma. Therefore, in general the rated leakage operation current of a leakage protector is adopted as 30 ma. Normally the leakage protector is not operated. When an electric shock occurs, if the shock current is greater than 30 ma the leakage protector will be operated to interrupt the power supply. The operation time of the leakage protector is generally less than 0.1 second, therefore the product of the current and the time is about 3 ma sec when the human body touches a live conductor, this product is much lower than the permissible limit of 30 ma sec so that safety can be assured.
The magnitude of the leakage current of a power line depends upon the aging conditions of the conductor of the power supply, the obsoleteness of the equipment, the amount of electric equipment being used, and the humidity of the atmosphere. The magnitude of the leakage current for each phase to the ground is variable. When the weather is dry, the resultant vector value of the leakage current to the ground for a single phase or three phase system will be much greater than 15 ma due to the increase of the capacity of the load. For the rainy season in the south of China and south of the Yantze River, the humidity in the air is high, the insulation of conductors and equipment is decreased, and the leakage current is much greater. Such as in the Guangtung Province of China, etc. The leakage current of mains and equipment with 100-200 ampere load capacity is generally more than 100 ma, and the variance of the leakage current between sunny days and rainy days is great. Therefore, even if the human body does not touch a live conductor, the above-mentioned leakage protector with 30 ma rated operation value of leakage current will be operated, thus the normal work order is disturbed, and a leakage current protector or a safety cut-off with its rated operation current greater than 100 ma must be adopted. When a protector with high rated operation current is used, since the normal leakage current is great, as soon as someone touches a live conductor the protector will be operated immediately when only a small shock current is superimposed on the original great leakage current and the power supply is interrupted. The effect for protecting the human body is good, but when the weather turns dry or the amount of electric equipment used is decreased, the leakage current is decreased to a small value. If someone touches a live conductor at this time, the protector will be operated only when a great shock current is superimposed on the relatively small leakage current, thus the shock current passing through the human body may be greater than 100 ma, causing danger even death from electric shock. The protector thus has no effect on protecting humans from electric shock. In order to solve this problem, the operation current of the protector can be made adjustable. The operation current is set according to the leakage current at a certain time, but as mentioned above, the leakage current is variable so that if the operation current of the protector is set according to a small leakage current, as soon as the leakage current is increased, the protector will malfunction, i.e. the protector will be tripped even if no electric shock occurs. If the operation current is set according to a great leakage current, when the shock current is just out of phase with the leakage current, the danger of shock exists and when the leakage current is reduced, the protector will be tripped only with greatly increased shock current so that it can not provide any protection. Another practice is to distinguish the leakage current from the shock current. This technique has two methods: the first is to judge from an abrupt swing of the amplitude of current, and the second is to judge from the characteristics of the current. The first method is a pulse type protector, in which current with an abrupt swing is considered a shock current and slow change current is considered a leakage current. The value of the operation current for electric shock is the vector sum of the shock current passing through the human body and the resultant leakage current with the original leakage current subtracted. When the live conductor touched belongs to a different power phase line, the operation current is different, therefore an insensitive operation dead zone exists, and when a person gets an electric shock in thsi dead zone, the safety of the human body can not be assured by the protector and the protector can easily malfunction by another reason such as the starting of equipment. A protector which distinguishes according to the waveform of the current was disclosed in UK patent application GB No. 2019677A published on Oct. 31, 1979. This publication recognized that the waveform of shock current is a peaked wave which changes slowly in the vicinity of zero and its frequency is the same as the frequency of the power supply. Therefore, a protector apparatus is provided, the operation of the protector apparatus being determined by analyzing parameters of the shock current such as the ratio of the third harmonic component to the fundamental component, the ratio of the peak current to the average current, and the ratio of the peak value to the slope at the zero crossing, to see if the ratio is greater than the rated value for operation occurring under the relatively small shock current with relatively great leakage current. There exists, however, a big third harmonic component usually in a load such as an electric machine, thus the waveform of the resultant current of the harmonic component and the leakage current is similar to that of the shock current so that malfunction occurs easily.